1. Field of the Invention
The present invention relates generally to image sensors, and more particularly to a CMOS image sensor with multiple protecting layers for preventing contamination of a photodiode that is an example photo-detector.
2. Background of the Invention
An image sensor is a semiconductor device for converting an optical signal into electric signals. Such an image sensor is categorized as a CCD (Charge Coupled Device) or a CMOS (complementary metal-oxide conductor) image sensor (CIS). CCDs and CMOS image sensors similarly use a photodiode for converting light into electric charge, but have different mechanisms for transferring the produced electric charge.
For example, the CCD generates a final digital signal by directly transferring the generated electric charge to an amplifier using a quantum well. In contrast, the CMOS image sensor transfers the generated electric charge by converting such electric charge into an analog signal such as a current or a voltage. The CCD is also called a “passive pixel sensor,” and the CMOS image sensor is also called an “active pixel sensor (APS)”.
CCDs have been widely used as high-quality image sensors having charge accumulation with high sensitivity. However, CCDs have high power consumption, and have difficulty in speeding up input optical data and in integrating peripheral circuits into a one-chip system.
A CMOS image sensor with a simpler circuit structure is more easily integrated into a one-chip system that is easily manufactured into a large image sensor system. In addition, the CMOS image sensor has low power consumption and high operating speed, and is inexpensive. However, since a photodiode of each unit pixel has a respective amplifier in the CMOS image sensor, each unit pixel may generate different signals for a same input light resulting in random noise.
FIG. 1 shows a circuit diagram of an active unit pixel of a conventional CMOS image sensor. Referring to FIG. 1, the unit pixel includes a photodiode PD, a transfer transistor TX, a reset transistor RX, a drive transistor DX, and a select transistor SX. Each of such transistors TX, RX, DX, and SX is a MOSFET (metal oxide semiconductor field effect transistor) for example.
The photodiode PD produces an electric charge in response to input light, and the transfer transistor TX is controlled to transfer such electric charge from the photodiode PD to a floating diffusion region FD. The reset transistor RX is controlled to periodically reset the floating diffusion region FD. The drive transistor DX and the select transistor SX are connected in series to form a source follower for buffering a signal corresponding to the electric charge stored at the floating diffusion region FD.
FIG. 2 shows a cross-sectional view of the photodiode PD and the transfer transistor TX of the conventional CMOS image sensor of FIG. 1. Referring to FIG. 2, the transfer transistor TX includes a gate structure 25 comprised of a gate electrode 22, spacers 24, and a spacer liner 23. The transfer transistor TX also includes a source/drain region 12 formed to a side of the gate electrode 22 within a semiconductor substrate 10. A gate oxide layer 21 is formed on the semiconductor substrate 10 under the gate structure 25 and over the photodiode PD.
The photodiode PD is formed at the other side of the gate electrode 22 within the semiconductor substrate 10. The photodiode PD includes a bottom diffusion region 14 having an opposite conductivity type as the source/drain region 12. The photodiode PD also includes a top diffusion region 16 having the same conductivity type as the source/drain region 12.
The photodiode PD receives light for generating an amount of electric charge corresponding to the intensity of such received light. Any defect of the diffusion regions 14 and 16 of the photodiode PD, such as metal contamination for example, causes current leakage in the photodiode PD resulting in image distortion such as white spots.
In addition, the conventional CMOS image sensor also includes a logic region in addition to the APS (active pixel sensor) region having an array of unit pixels. The logic region is for further processing the signals output by the unit pixels. High signal processing speed by the logic region is desired, and thus, the source/drain regions of a transistor within the logic region include metal silicide.
In that case, a photodiode protecting layer is formed over the photodiode PD for preventing metal silicide from reaching the photodiode PD in the APS region. However, such a photodiode protecting layer may be partially damaged during the fabrication process resulting in metal contamination of the photodiode PD through damaged parts of the photodiode protecting layer. Such contamination undesirably results in leakage current within the photodiode PD causing white spots in a captured image.